Methods involving separating materials by employing membranes have recently been developing and have been drawing great attention in various fields of technology and industry, including waste water disposal, food industry and medical applications. These methods include microfiltration, ultrafiltration and dialysis, and are selectively employed to suit the particle sizes and materials floating, dispersed or dissolved in an aqueous medium. While different types of membranes are, of course, used for different purposes of separation, all the membranes are required to have a high rate of permeation to an aqueous medium, to be highly capable of removing the materials floating, dispersed or dissolved therein, and to have a high degree of mechanical strength and chemical resistance.
For such requests, hollow fiber separation membranes of, for example, cellulose or cellulose acetate are proposed. However, such membranes have faults as follows: the structure of the membrane tends to become excessively minute; since it is difficult to form fine pores of a suitable size for the hollow fiber type ultrafiltration membrane, the manufacturing process tends to become complicated; consequently, the strength of the hollow fiber is excessively reduced and its quality is not constant. In this connection, the ion-crosslinked hydrous PMMA membrane which is obtained from a mixture of methyl methacrylate copolymer containing sulfonic acid group and methyl methacrylate copolymer containing quarternary nitrogen group, is already proposed in U.S. Pat. No. 4,439,322.
The proposal makes it possible to obtain hollow fiber type ultrafiltration membranes of various grades which are excellent in performance and mechanical characteristics, but some problems are involved in the proposal because it takes the introduction of ion-crosslinking as its essential condition. Examples of the problems are as follows: since dissolving conditions of the spinning solution are not stable, the greatest care must be given to obtain homogeneous dissolution; meshes of the filter tend to be packed during spinning, namely, the pack-life is short; the greatest care is necessary for stable storage of the spinning solution; since ion balance has a large influence on the performance and mechanical characteristics, much care must be taken; the greatest consideration must be given to control the copolymer because ion distributions in the copolymer have an effect on the performance and others.
In order to solve the aforesaid problems, the present invention has found a product capable of being constantly prepared at low cost by developing a technique in which a PMMA polymer containing no ion pair is applied as the raw polymer.
Typical forms of selective semipermeable membranes include three types such as flat membrane, tubular membrane, and hollow fiber type membrane, which have an individual feature of application and device depending on each form. Among them, the hollow fiber type membrane has such merits that a large membrane area is obtainable, a small-sized device is available, and loss of the membrane is less and the costs are low, in the manufacture of devices and modules.